Granulosa cell differentiation which occurs during the final stages of ovarian follicular maturation is characterized by three major functional alterations, these being 1) induction of cell surface receptors for luteinizing hormone, 2) increased capacity to produce progesterone and 3) increased ability to produce estradiol in the presence of androgen substrate. Available evidence strongly suggests that each of these developmental changes is controlled by follicle stimulating hormone. Ecperiments proposed herein are designed to investigate the cellular events required for FSH to initiate granulosa cell differentiation. Using immature rats as a model system, the requirements for either DNA, RNA or protein synthesis will be inves tigated during the twenty-four hour interval in which follicle stimulating hormone effects these developmental changes. In addition, the feasibility of using somatic cell hybridization to obtain long-term tissue culture cell lines of rat granulosa cells for detailed in vitro experimentation will be investigated. For this purpose, rat ovarian tumor cells will be fused with freshly isolated rat granulosa cells using Sendai virus with the purpose of obtaining cell hybrids which retain the growth properties of the tumor cells and the specialized properties of the primary cells. Hybrid cells will be monitored for 1) their ability to bind radioiodinated FSH or HCG to determine if these cells possess gonadotropin receptors and 2) their ability to produce steroid hormones under basal conditions or in response to stimulatory agents such as gonadotropins and cyclic nucleotides. Results from these experiments may provide a general mechanism to obtain clonal cell lines derived from endocrine tissues which retain their specialized properties.